I have found what works best is make her jealous. Like drop little subtle things, nothing serious, but enough to make her worried. Start talking to other more attractive girls. Because there's one thing you have to remember, no matter how much they won't admit it, girls like assholes. Now I'm not saying like straight up be a dick, but make fun of her, make plans the back out, that kind of stuff. The key here is to make her jealous.